Since the earliest days of aviation, particularly military aviation, pilots have preferred to look forward in the cockpit while flying. This preference led to the development of the head-up display (HUD), which presents information to the pilot without requiring him/her to look down or away. The HUD is essentially a transparent or semi-transparent display that allows the pilot to view, for example, navigational or weapons-aiming information, while looking straight ahead. More recently, helmet-mounted displays (HMD) have been developed that are mounted on the pilot's helmet directly in front of one or both of the pilot's eyes. The HMD is essentially a smaller HUD that allows the pilot to view the same navigational and/or weapons-aiming information as the HUD.
In order for any ground coordinated information to be presented to the pilot correctly, an HMD must be provided with the position and angle of the pilot's head. Thus, the HMD typically operates in conjunction with a helmet tracking system (HTS) that determines the position and angular orientation of the pilot's helmet with respect to a predefined reference, such as the aircraft axes. The helmet tracking system tracks the movement of the pilot's helmet in six degrees of freedom: X-axis, Y-axis, Z-axis, roll, pitch, and yaw. This tracking ensures that the information displayed on the HDM is correctly aligned in space and/or is precisely superimposed over external objects being viewed by the pilot.
It is important that helmet tracking systems be highly accurate and have fast dynamic response, particularly in armed forces applications, as a military pilot's head movements may be extremely rapid. However, the above criteria are often mutually exclusive so that existing helmet tracking systems tend to be a compromise between the two objectives. For example, inertial systems based on gyroscopes have a very fast dynamic response, but the accuracy of these systems is limited by drift over extended periods of operation. On the other hand, helmet tracking systems that are based on magnetic sensors are very accurate, but do not provide a fast enough dynamic response because of the slow settling times of the magnetic fields. Moreover, both inertial systems and magnetic systems tend to be relatively expensive due to the high cost of the complex components used in each system.
Accordingly, what is needed is a helmet tracking system that overcomes the deficits and shortcoming of existing systems.